The invention relates to a novel gene, EPM2A, that is involved in Lafora""s disease; the protein, Laforin, encoded by the gene; and methods of diagnosing and treating Lafora""s disease.
The epilepsies constitute one of the most common neurological disorders affecting 40 million people worldwide (1). Within the spectrum of epileptic syndromes is a group of heterogeneous inherited disorders named the Progressive Myoclonus Epilepsies (PME) in which progressive neurological decline and worsening primarily myoclonic seizures follow an initial period of normal development (2,3,4). Lafora""s disease (LD) is an autosomal recessive and genetically heterogeneous form of Progressive Myoclonus Epilepsy characterized by polyglucosan inclusions seizures and cumulative neurological deterioration. The onset occurs during late childhood and usually results in death within a decade of first symptoms. With few exceptions, patients with LD follow a homogeneous clinical course (4) despite the existence of genetic locus heterogeneity (5). Biopsy (or autopsy) of various tissues including brain, liver, muscle, and skin reveals characteristic periodic acid-Schiff positive polyglucosan inclusions (Lafora bodies) (6-9). Substantial biochemical and histological studies of these bodies suggest LD is a generalized storage disease (8,10,11), but the presumed enzymatic defect remains unknown.
Linkage analysis and homozygosity mapping initially localized a Lafora""s disease locus (EPM2A) to a region at chromosome 6q23-q25 bounded by the genetic markers D6S1003 and D6S311 (12,13). However, there is a need in the art to more clearly define the region(s) mutated in Lafora""s disease to allow for the development of accurate diagnostic assays for Lafora""s disease. More specifically, there is a need to sequence; the gene associated with Lafora""s Disease and to identify mutations and/or deletions in the gene that are causative of Lafora""s Disease.
The present inventors have identified a novel gene, EPM2A, that is deleted or mutated in people with Lafora""s disease. Using a positional cloning approach the inventors have identified at chromosome 6q24 the EPM2A gene that encodes a protein with consensus amino acid sequence indicative of a tyrosine phosphatase. Accordingly, the present invention provides an isolated nucleic acid molecule containing a sequence encoding an active catalytic site of a protein tyrosine phosphatase which is associated with Lafora""s disease.
In one embodiment of the invention, an isolated nucleic acid molecule is provided having a sequence as shown in SEQ.ID.NO.:1 or FIG. 13.
Preferably, the purified and isolated nucleic acid molecule comprises:
(a) a nucleic acid sequence as shown in SEQ.ID.NO.:1 and FIG. 13, wherein T can also be U;
(b) nucleic acid sequences complementary to (a);
(c) nucleic acid sequences which are homologous to (a) or (b);
(d) a fragment of (a) to (c) that is at least 15 bases, preferably 20 to 30 bases, and which will hybridize to (a) to (d) under stringent hybridization conditions; or
(e) a nucleic acid molecule differing from any of the nucleic acids of (a) to (c) in codon sequences due to the degeneracy of the genetic code.
Fourteen different mutations in EPM2A in 24 families have been found that co-segregate with Lafora""s disease. These alterations would be predicted to abolish or cause deleterious effects on the protein product, Laforin, resulting in the primary defect in a large portion of patients with the disease. Accordingly, the present invention provides a method of detecting Lafora""s disease comprising detecting a mutation or deletion in the EPM2A gene in a sample from a mammal. A mutation can be detected by sequencing the EPM2A gene, in particular in the region in the gene between markers D6S1003 and D6S1042, in a patient and comparing the sequence to the wild type EPM2A sequence shown in FIG. 13 to determine if a mutation or deletion is present. A mutation or deletion can also be detected by assaying for the protein product encoded by EPM2A, Laforin.
Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples while indicating preferred embodiments of the invention are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.